1. Field of the Invention
The present invention relates to an evacuation apparatus for creating a vacuum and also to an evacuation method which is carried out by operating an evacuation apparatus.
2. Description of the Prior Art
A typical conventional turbomolecular pump will first be explained with reference to FIG. 5.
A conventional turbomolecular pump, which is generally denoted by reference numeral 1, includes a motor 2, a motor shaft 3 for transmitting the rotational force that is derived from the motor 2, a rotor 4 which is secured to the motor shaft 3, a plurality of rotor blades 5 which are fixed to the rotor 4, a plurality of stator blades 6 each disposed between a pair of adjacent rotor blades 5, a spacer 7 having the stator blades 6 attached thereto, a casing 10 which is provided with a suction port 8 and an exhaust port 9, and a protective net 11 for protecting the rotor and stator blades 5 and 6. In operation, the motor 2 is driven to rotate the rotor blades 5 at high speed in an atomosphere that has sufficient vacuum present for a molecular flow to be available, thereby sucking gas molecules from the suction port 8, compressing the gas at a high compression ratio and moving the gas toward the exhaust port 9, and thus producing a high vacuum.
The above-described conventional turbomolecular pump suffers, however, from the following problems. Namely, the gas exhausting performance of the pump depends on the molecular weight of the gas that is removed thereby. When a gas having a low molecular weight is required to be removed, the gas exhausting performance is considerably lowered. The lower the compression ratio, the lower the gas exhausting performance. The blade speed ratio C as a parameter representing the compression ratio is expressed as follows: EQU C=V/Vm
(wherein V is the peripheral speed of the rotor blades and Vm is the maximum probability speed of gas molecules).
The maximum probability speed Vm of gas molecules is expressed as follows: EQU Vm=.sqroot.(2KT/M)
(wherein M is the molecular weight of the gas, K is Boltzmann's constant, and T is the absolute temperature of the gas).
As will be clear from these expressions, the lower the molecular weight M of the gas, the higher the maximum probability speed Vm of the gas molecules and the lower the blade speed ratio C. Therefore, when a gas having a low molecular weight is required to be removed, the gas exhausting performance is low. When the gas exhausting performance is low, many problems are likely to occur in the actual operation of the turbomolecular pump.
Among the problems associated with gases having low molecular weights, the existence of water vapor, in particular, adversely affects the gas exhausting performance of the pump. In a system wherein a part of the system that is provided with a turbomolecular pump is open to the atmosphere and air flows into the system, the greater part of the residual gas under a vacuum of about 10.sup.-4 Torr to 10.sup.-10 Torr (10.sup.-4 mmHg to 10.sup.-10 mmHg) which is produced by the turbomolecular pump is water vapor. The residual water vapor has adverse effects on the degree of vacuum attained and the vacuum environment.
In a case employing a cryo-vacuum pump that employs a helium refrigerator and a heat exchanger which provides ultra-low temperatures of from about 15.degree. K. to about 20.degree. K., the gas exhausting characteristics with regard to water vapor are improved and it is therefore possible to a certain extent to overcome the above-described problems. However, such a cryo-vacuum pump involves the following problems:
(1) Since a refrigerator for ultra-low temperatures is used, it takes a long time to start and suspend the refrigerator. PA1 (2) Since the pump is a capture type pump, i.e. it freezes and traps most gas molecules, it must be regenerated for a long period every time a predetermined load running is completed. PA1 (3) Since the sublimation temperature differs depending upon the kind of gas molecules, various kinds of gas molecules are separated from each other and successively discharged from the pump at high concentrations as the temperature of the heat exchanger rises during a regenerative operation, and it is difficult to treat various kinds of gases which are discharged separately. In particular, in semiconductor manufacturing processes, toxic, highly-corrosive, explosive and combustible gases, for example, monosilane (SiH.sub.4), hydrogen fluoride (HF), etc., are used that are diluted with inert gases such as nitrogen (N.sub.2), helium (He), etc., and it is therefore extremely difficult to cope with the separate discharge of these various kinds of gas.